Spread spectrum communication is advantageous in communication applications requiring high reliability in a noisy environment. According to Shannon's theory, a widened spectrum can lower the requirement for a high signal-to-noise ratio, which indicates that a weak signal can be transmitted and detected by using the spread spectrum communication technology. In order to spread the spectrum, a high-speed pseudorandom noise (PRN) code is often used to modulate a narrow-band signal to generate a wide-band signal. To communicate data, the wide-band signal is modulated by a message data stream. The message data rate is usually much lower than the PRN code symbol or “chip” rate, and the data and code-chip signal edges are usually synchronized.
Message data from a spread spectrum signal, such as a GPS signal, can be retrieved by first converting the received signal down to a lower frequency by multiplying it with a locally generated carrier signal. The local carrier signal may be generated by a properly tuned local oscillator. If the frequency and phase of the local carrier signal are the same as those of a received original narrow-band carrier, then the multiplier output signal from multiplication of the received signal and the local carrier signal will be a bipolar wide-band data stream. This bipolar wide-band data stream is the product of the bipolar PRN code and message data sequences. The PRN code is then removed by multiplying the wide-band data stream with a locally generated PRN code that is time aligned with the received PRN code. Thus, the message data can be obtained. The above-mentioned process is a signal despread process.
GPS signals are spread spectrum signals broadcasted by the GPS satellites on L1, L2, and L5 frequencies. Current commercial GPS receivers generally use the L1 frequency (1575.42 MHz). There are several signals broadcasted on the L1 carrier: coarse/acquisition(C/A) code, P code and the navigation data. The detailed information of the satellite orbit is contained in the navigation data. The C/A code is mainly used by civilian receivers for positioning purposes. The C/A code is used to determine a pseudo-range (the apparent distance to the satellite), which is then used by the GPS receiver to determine a position. The C/A code is a type of the pseudorandom noise (PRN) code, the functionality of which has been previously described. A radio frequency signal coded by the C/A code becomes a spread spectrum signal. Each satellite has a unique C/A code and repeats the C/A code over and over again. The C/A code is a sequence of zeros and ones (binary). Each zero or one is known as a “chip”. The C/A code is 1023 chips long, and it is broadcasted at 1.023 Mega-chips per second, i.e., the repetition of the C/A code lasts 1 millisecond. It is also possible to regard each chip as having two states: +1 and −1.
A set of data collected by a GPS receiver usually contains signals from several satellites. Signals travel from different satellites through different channels. Usually, the GPS receiver simultaneously processes the signals from several channels. Each signal has a different C/A code with a different starting time and different Doppler frequency shift. Therefore, to find the signal of a certain satellite, GPS receivers traditionally conduct a two dimensional search, checking each C/A code with different starting time at every possible frequency. “Different starting time,” as used herein, can be interpreted as the result of the phase delay of a C/A code. In a GPS receiver, an acquisition method is employed to find the beginning of the C/A code and carrier frequency, in particular, the Doppler frequency shift of the signal. To test for the presence of a signal at a particular frequency and C/A code delay, the GPS receiver is tuned to the frequency, and the incoming signal is correlated with a known PRN code delayed by an amount corresponding to the time of arrival. If no signal is detected, the search continues for the C/A code with a next possible delay. Traditionally, each possible delay of the C/A code is obtained by shifting the C/A code by half a chip. Since a C/A code comprises 1023 chips, 2046 delay possibilities may need to be checked for a fixed frequency. After all delay possibilities are checked, the search continues to a next possible frequency. Because thousands of frequencies and code delays may need to be checked, the speed of the acquisition process is highly important.
FIG. 1 illustrates a prior art block diagram of a GPS receiver 100. In general, a GPS receiver includes two parts: RF (radio frequency) front end module 101 and base-band signal processing module 103. The GPS signals transmitted from the GPS satellites are received from an antenna 102. Through a RF tuner 104 and a frequency synthesizer 105, a received signal (also known as input signal) is converted from the GPS signal (a radio frequency signal) to a signal with a desired output frequency. Then, an analog-to-digital converter (ADC) 106 digitizes the converted signal at a predetermined sampling frequency. The converted and digitized signal is known as intermediate frequency (IF) signal. The IF signal is then sent to the base-band signal processing module 103, which includes several signal processing stages. The IF signal is sent to an acquisition module 110 where Doppler frequency shift search and C/A code phase shift search are conducted, as described above. During the acquisition stage, the integration of the IF signal is completed by performing correlation based on the IF signal and C/A code. A tracking module 112 is capable of tracking the GPS signal through IF signal by using a carrier tracking loop and a code tracking loop, thus, obtaining the navigation data contained in the GPS signal. Then, a navigation data calculation module 114 and a position calculation module 116 may utilize the navigation data to calculate the user's position.
To achieve a better performance, parallel correlators are conventionally employed to conduct parallel searches. However, using a large number of the parallel correlators demands large logic resources and requires high correlation frequency thereby making it hard for the acquisition process to realize in an Application Specific Integrated Circuit (ASIC) if no optimization design is adopted. Thus, it is to an improved acquisition module that enables parallel correlation the present invention is primarily directed.